High brightness light emitting diode (LED) light sources have only recently become available at performance levels suitable for challenging high intensity applications in UV curing. There has been a significant increase in the use and value of UV cured materials in the manufacturing process due to significantly higher production throughput afforded by the extremely fast curing of materials in comparison to curing by other conventional means including heat, non-photoinitiated chemical interactions of adhesives, evaporation of volatiles, and the like. This improvement in process time has significant value to the manufacturing community. Imaged UV curing in the case of maskless lithography saves considerable cost in eliminating the need to produce the mask, in addition to the time savings. In the case of three dimensional (3-D) printing, faster cure times result in faster build times which when combined with new high performance curable UV resins enables true 3-D digital printing for manufacturing, an area of technology which is growing quickly.
Conventional UV curing devices typically utilize short arc mercury lamps, xenon lamps, lasers and more recently, systems incorporating pre-packaged high brightness LEDs. Conventional arc lamps suffer from significant arc flicker resulting in the need to homogenize the light which in typical non-Etendue preserving designs reduces radiance (optical power per unit area per unit solid angle [W/cm2/sr]). Arc lamps also suffer from poor lifetime and rapidly decreasing output as a function of time with lifetimes in the hundreds to low thousands of hours at best. This results in added system cost and maintenance expense relative to LED sources which are more stable in output in both the short and long term, and characterized by lifetimes of tens of thousands of hours given proper attention to thermal design. In the last several years laser diodes have been used to replace arc lamps, however, laser diodes are extremely expensive compared to both arc lamps and LEDs, and suffer from image artifacts due to the high temporal and spatial coherence relative to LEDs. Conventional implementations using LEDs, however, suffer from relatively low radiance and lifetime due to limitations of their optical, mechanical and thermal designs.
Prepackaged LEDs are defined as devices comprising an LED die or die array sitting on top of one or more thermally and electrically conductive materials. The thermal impedances of these multiple interfaces leads to a cumulative total thermal impedance that results in high LED junction temperature, thereby degrading LED output and life. Examples of prepackaged UV LED devices include those offered by the SemiLeds® and Nichia® product lines.